This invention relates to a wrapping apparatus in which articles are wrapped in a stretchable film or the like on a tray-by-tray basis.
As one type of wrapping apparatus in which article accommodated in individual tray-shaped containers (hereinafter referred to simply as trays) are each wrapped in a transparent stretchable film, there has heretofore been a wrapping apparatus in which a predetermined cut length of film fed by a film feeding mechanism is extended at a predetermined portion of the wrapping apparatus, an article to be wrapped is raised into engagement with the extended film from therebelow by an elevator mechanism, and the edges of the film are folded under the bottom of the article by left and right folding plates and a front or rear folding plate, having respective independent drive means, in such a manner that the left and right edges of the film are first folded under the bottom of the tray and the front or rear edge of the film is then folded under the bottom of the tray by starting the left and right folding plates in advance of the starting of the front or rear folding plate, thereby wrapping the article in the film with the film edges being beautifully folded under the bottom of the tray.
However, since trays vary in size and shape, the final appearance of the wrapped articles depends upon the type of tray and the prior art suffers from problems, for example, wrinkles in the film covering the tray surface.
To cope with the problems, a wrapping apparatus disclosed in the specification of Japanese Patent Application Laid-Open Publication (KOKAI) No. 61-190408 is arranged such that the start timing of left and right folding members that is in advance of the start timing of a front folding member is controlled in accordance with the cut length of film that is set depending upon the type of a tray selected.
However, attention has recently been paid to the problem regarding the food hygiene of vinyl chloride film and consequently polyethylene film has increasingly been used in place of vinyl chloride film.
Polyethylene film is lower in the level of adhesion to the tray and higher in the level of restoring force produced when stretched than vinyl chloride film. Therefore, when a tray which has a high length-to-width ratio (e.g., a slender tray) is to be wrapped in a film having different properties from those of vinyl chloride film, since the front (rear) folding member folds the front (rear) edge of the film under the bottom of the tray after the left and right folding members have folded the left and right edges of the film deep under the bottom of the tray, the film is forced toward the center of the tray, resulting in wrinkles extending radially from the center.
In addition, since an unnecessarily high tension is applied to the film, the tray may become warped. Since the restoring force of the film when pulled is strong and the level of adhesion to the tray is low, when the grippers grasping both edges of the film are released, wrinkles are produced by restoration of the warped tray and slacking of the film.
Thus, the prior art wrapping apparatus has heretofore been incapable of coping with the problems when a film having different properties from those of a specific one is employed. The applicant of the present application has already developed and filed "Film Folding Mechanism of Wrapping Machine" (Japanese Patent Application No. 62-211707). With this mechanism, however, the above-described problems have not yet been satisfactorily solved.